1. Field of the Invention
The present invention relates to a display device, and more particularly, to a pixel structure for use in an electrowetting display device, an electrowetting display device comprising such a pixel structure.
2. Descriptions of the Related Art
With the rapid advancement of science and technology, various convenient and portable electronic devices, such as mobile phones, personal digital assistants (PDAs), digital cameras, digital video cameras or the like, are becoming increasingly diversified in terms of specifications and functionalities. Specifically, the continuous development of display technologies for portable electronic devices has catered to the various needs of consumers.
Because portable electronic devices are required to have a small volume and light weight to provide good portability, such devices must also conserve power to prolong the endurance of batteries, thereby obviating inconveniences associated with frequent battery recharging. As an effective technology of reducing power consumption of portable electronic devices, a reflective display employs light from an ambient environment as a light source and reflects the light with a reflective sheet located beneath a display panel. Because such a display does not rely on self-illumination, there is no need of a backlight source and a backlight panel. As a result, both the power consumption and weight of the device are reduced. Moreover, the decreased thickness and simpler structure allows the device to be manufactured more easily and at a lower cost, thereby reducing the price of the product for consumers.
Among the display devices that employ external light sources, the electrowetting display, devices demonstrate satisfactory optical characteristics. The electrowetting display devices not only feature a light weight and thin profile, but also allow for a very fast switching operation of the pixel. As a result, movies can be played more smoothly. Therefore, the electro-wetting display devices are highly desired.
FIGS. 1a and 1b are schematic views of a pixel structure 1 of a conventional reflective electrowetting display device. The pixel structure 1 has an opaque substrate 11 adapted to reflect light entering the pixel structure 1 from an ambient environment as the light source of the display device. A reflective electrode layer 12 is formed on the substrate 11 and a transparent electrode layer 14 is formed above the substrate 11 and the reflective electrode layer 12. The reflective electrode layer 12 is controlled by a thin film transistor (not shown) and further has a hydrophobic layer 13 formed thereon. A metallic reflective material (e.g., aluminum or aluminum alloy) is coated on either the upper or lower surface of the reflective electrode layer 12 to reflect the ambient light. An opaque non-polar liquid 15 and a transparent polar liquid 16 are contained between the transparent electrode layer 14 and the hydrophobic layer 13. The hydrophobic layer 13 is formed from a transparent insulation material. The non-polar liquid 15 is typically an oil-based ink, while the polar liquid 16 is typically water. The hydrophobic layer 13 is typically formed of a material selected from the teflon series.
As shown in FIG. 1a, when no voltage is applied across the two electrodes, the attractive force of the hydrophobic layer 13 to the polar liquid 16 is small compared to the surface tension of the polar liquid 16, so it is difficult for the polar liquid 16 to be adsorbed thereon. In contrast, the hydrophobic layer 13 exhibits more affinity to the non-polar liquid 15, which makes it possible for the non-polar liquid 15 to be dispersed throughout the hydrophobic layer 13. Accordingly, when light enters the pixel structure 1, it will be partially or entirely absorbed by the dispersed non-polar liquid 15 to present the color of the non-polar liquid 15.
As shown in FIG. 1b, when a voltage is applied across the two electrodes, a change in the dielectric characteristics and surface characteristics occurs to the hydrophobic layer 13 under the influence of the resulting electric field between the two electrodes, causing the hydrophobic layer 13 to exhibit more affinity to the polar liquid 16 instead. As a result, when a voltage is applied, the polar liquid 16 expels the non-polar liquid 15 to the sides of the pixel due to the attraction of the hydrophobic layer 13, of which the surface energy is changed. As a result, the pixel structure 1 presents the color of the substrate 11.
In a practical display device, a white substrate 1 may be used and a multiplicity of pixels using inks of different colors may be combined. By using electrodes to control the respective pixels, the ink can be dispersed throughout, or contracted to a corner of, the respective pixels to yield a visual effect like a printed colorful paper. Such an electrowetting-based display exhibits superior performance, and switches between the reflections of the white color and other colors at a speed which is sufficiently fast for movie playing. Furthermore, such a display saves considerable power and is made to be very flat and thin.
However, because the ink layer in the electrowetting display of such a structure is not completely transparent, the light of a color similar to the ink will still appear above the ink when the ink is contracted to a corner of the pixel, causing degradation in the contrast ratio demonstrated by the pixel when switched between different switching states. In reference to FIG. 1c, a black matrix 17 is additionally formed on the transparent electrode layer 14 in the conventional pixel structure 1 in such a way that the black matrix 17 is positioned and sized to completely shield the contracted ink, thereby improving the contrast ratio of the display device. However, because the transparent electrode layer 14 is formed on the counter substrate opposite the substrate 11, precise alignment and package of the substrate 11 and the counter substrate with the transparent electrode layer 14 and the additional black matrix 17 is necessary. Moreover, when the reflective electrode layer 12 is coated with a reflective material on the upper or lower surface thereof, an additional patterning process must be performed using an appropriate photomask to have the reflective material patterned according to the locations of the TFTs, which would add to the complexity and cost of the manufacturing process.
In view of this, it is highly desirable in the art to provide a pixel structure for use in an electrowetting display that provides an improved contrast ratio of pixels as well as decreased manufacturing difficulty and costs.